A radio frequency (RF) integrated circuit may include multiple transistor dies that are placed in an integrated circuit package by a die attach machine. A robotic bonding tool may then be used to wire bond the dies to other circuit elements within the package, and to leads of a package leadframe. Such a tool generally includes a surface/wire-feed detection system that detects bond pads or other bond sites of a given die, and determines the height coordinates of these bond pads. The other circuit elements in an RF integrated circuit may include, for example, tuning capacitors.
The wire bonding of the various circuit elements may create several differently-shaped wire bond profiles, depending on the placement of the various circuit elements to be connected by wire bonds. A wire bond profile may be characterized as a side or profile view of a wire extending from a first bond site to a second bond site. In an RF integrated circuit, the wire bonds may carry high frequency signals. Certain types of RF integrated circuits, such as RF power transistors, are tuned through these wire bond profiles. Therefore, it is important for these wire bond profiles to achieve a desired shape for optimal RF performance.
The two major wire-bonding processes used for electronic package interconnects are wedge bonding and ball bonding. The wedge-bonding process has traditionally been used to form the package interconnects of RF integrated circuits due to its ease in forming the wire bond profiles necessary for optimal RF performance. While ball bonding provides a more economical and robust process than that of wedge bonding, the inability of traditional ball bonders to achieve the necessary wire bond profiles has created an overwhelming bias against using modern ball-bonding processes for wire bonding RF integrated circuits.
Traditional ball bonders typically incorporate a single reverse motion of the bonding tool during wire bond profile formation so that the completed wire bond profile may have a section of wire that extends vertically for a considerable distance above the ball at the first bond site. However, traditional ball bonders have difficulty in precisely controlling the amount of wire in the wire bond profile. For example, traditional ball bonders do not have a sufficient range of z-axis motion to enable all the requisite wire to be fed out above the first bond site for high wire bond profiles. Consequently, the wire continues to be fed out during the approach to the second bond site. As the bonding tool moves away from the first bond site, the drag of the wire through the tool increases, which introduces variability in the amount of wire length in the wire bond profile. This is unacceptable for RF applications.
Thus, wire bond profiles with vertical extensions above the first bond site are skewed or bowed away from the second bond site, thereby deviating from the desired wire bond profile shape. This bow away from the second bond site causes increased cross coupling with other wire bonds in the RF integrated circuit. The inability of the traditional ball bonder to produce desired wire bond profiles also prevents crossing points of wire bond profiles from occurring at a point where the wires are substantially perpendicular. These deviated crossing points also cause increased cross coupling in the RF integrated circuit.
Traditional ball bonders are generally only able to perform ball-bonding operations from a die to packaging or leadframe leads, since the wire bond terminations are too harsh for a die surface. These terminations are traditionally similar to wedge bonds. Additionally, the ball size associated with the traditional ball bonder is very large, typically four times the wire diameter, requiring the use of larger bond pads. Since optimal RF performance often requires minimal bond pad size, the larger ball sizes are also a factor in favoring the use of wedge bonding instead of ball bonding in the fabrication of RF integrated circuits.
A ball-bumping technique of modern ball bonders may allow wire bond terminations to be placed on a primary die and capacitors without being too harsh for the die surface. Ball bumping was developed for ball bonding to allow chip-to-chip jumper wires to be bonded. Additional recent developments in commercially-available ball bonders include improvements such as the ability to perform two separate reverse motions, ball size reduction, and wire length control. Nonetheless, a need remains for further improvements in ball-bonding techniques, particularly in RF integrated circuit applications.